Microlithography is used to produce microstructured components, such as, for example, integrated circuits or LCDs. The microlithography process is performed in a so-called projection exposure apparatus, which has a lighting device and a projection lens. The image of a mask (=reticle) illuminated via the lighting device is in this case projected via the projection lens onto a substrate (for example a silicon wafer) which is coated with a light-sensitive layer (photoresist) and is arranged in the image plane of the projection lens, in order to transfer the mask structure onto the light-sensitive coating of the substrate.
In projection lenses designed for the EUV range, i.e. in the case of wavelengths of approximately 13 nm or approximately 7 nm, for example, mirrors are used as optical components for the imaging process owing to a lack of availability of suitable light-transmissive refractive materials.
These mirrors have a mirror body (for example consisting of a glass ceramic) and a reflective coating. Owing to the small size of the structures to be imaged, stringent requirements are placed on the imaging properties of the projection system. Imaging errors can only be tolerated to a very small extent.
Mirror modules for use in the EUV range with mirrors which cannot be regulated within all six degrees of freedom via actuators in respect of their position (“unregulated mirrors”) are generally designed in such a way that the mirror is connected to an objective structure via a plurality of coupling elements. The coupling elements substantially have the task of compensating for different thermal expansions owing to different coefficients of thermal expansion (CTE) between the mirror and the objective structure and transmitting the holding forces required for connecting the mirror and the objective structure. These coupling elements can be manufactured from different materials. Special metallic coupling elements provide relatively low coefficients of thermal expansion but they are more magnetostrictive in comparison with non-metallic coupling elements.